Interspinous implants comprising an implant body designed to be inserted between the spinous processes of adjacent vertebrae and limit their movement with respect to one another are well known. While the implants are designed to restrict the relative movement of the adjacent vertebrae, they do not prevent movement all together, so are a good alternative to fusion for many conditions. Such implants have been successfully used for a number of years to stabilise the spine and provide disc support for patients suffering from various conditions known generally as degenerative disc disease. These implants represent a very useful tool in spinal surgery, especially since the procedure is reversible and leaves other options open in the future.
U.S. Pat. No. 6,626,944 discloses a basic version of one such implant, which has a implant body with recesses that define an interspinous portion designed to contact spinous processes of adjacent vertebrae. The implant has a band that is attached on one side of the implant body and, in use, is fed around each spinous process then secured back to the implant body, thereby effectively tying the spinous processes to the implant to restrict relative movement.
Installing this type of implant necessitates a surgical procedure during which the interspinous ligament connecting the two spinous processes between which the implant is to be inserted is extracted and openings for inserting bands are formed in the superior and inferior interspinous ligaments. Before the implant is inserted, one end of the band is attached to the implant body. Once the implant body is in place, the other end of the band is fed around one of the spinous processes. When a single band is used to secure the implant body, the band is then fed around the adjacent spinous process and attached back to the implant body. A similar process can be used with two bands.
Although installing the implant body between the two spinous processes is relatively quick and easy, attaching the band to the implant body to secure the implant in place is much more challenging. This is mainly because the incision and muscle retraction provides only a very restricted area in which to work. This part of the procedure can be time consuming and complicated.
In U.S. Pat. No. 6,626,944 the band is secured to the implant body by passing the band through a conduit in the implant body, passing a sleeve over the band and crimping the sleeve to the band to prevent the band passing back through the conduit. Alternatively, the band can be secured by passing it through the conduit and then tying the band in a knot around the conduit. However, in practice crimping or tying knots in such a restricted space is very difficult and fiddly for the surgeon. In addition, the crimps or knots could work loose over time, which would mean that the implant is no longer secured to the spinous processes. If this happened, the implant would be ineffective, with the consequential negative effects on the patient.
A similar implant is described in EP 1367954 which aims to address some of these problems by providing “removable fixing members” separate from the implant body. These are designed to be attached to each side of the implant body, once this has been located in place between the adjacent spinous processes during a surgical procedure. Once the implant body is in place, a band is passed around each of the spinous processes. The end of each band is then passed though an aperture in the removable fixing member, the removable fixing member is rotated about 360° anti-clockwise relative to the implant body to ensure the band passes twice between the member and the implant body, and the fixing member is attached to the body of the implant in situ using abutments on the fixing member which snap into correspondingly shaped housing in the implant body. The band is then tightened to secure the implant in place.
The implant described in EP 1367954 has been marketed as the “Wallis® Implant”. It has been used in practice and has been successful in treating and preventing various different conditions. See, for example, “Mechanical supplementation by non-rigid fixation in degenerative interspinous lumbar segments: the Wallis system” by J Senegas, Eur Spine J (2002) 11 (Suppl. 2):S164-S169.
However, the removable fixing members in EP 1367954 are, in practice, still extremely fiddly to use. It is particularly difficult to attach the removable fixing members to the implant body as this requires the application of lateral force to push the abutments of the removable fixing members so that they snap into the implant body housing, where lateral force is difficult to apply due to the space restrictions.
In addition, the surgeon must ensure that the band is passed in the correct direction through the removable fixing member, and that the removable fixing member is rotated 360° anti-clockwise relative to the implant body before attachment. This has proved to be counterintuitive and adds a significant degree of room for human error by the surgeon.
Once the removable fixing members of EP 1367954 have been attached to the body of the implant, the band must be tightened. However, if the band is pulled away from the implant body, or if the band is over-tightened, this has the effect of pulling the removable fixing member away from the implant body and can actually cause it to become detached from the implant body. Since tightening is the last stage of fitting the implant the surgeon may not notice, and accordingly the implant might be left in the patient with the removable fixing member not properly attached to the implant body. This means that the implant is not properly secured in place so will be less effective and may fail.
The present invention aims to address some of the problems encountered with current interspinous implants. In particular the present invention aims to provide an interspinous implant that is simpler and easier to fit, that reduces the possibilities for human error, and that provides a better and safer way of securing the implant in place.
As noted above, once the bands are in place around the spinous processes and have been attached to the implant body they must be tightened. Further problems with existing implant installation concerns the method of tightening. It is fiddly and difficult to manually pull the band to tighten it or to use a surgical instrument that has not been designed for this purpose, such as a pincers or pliers.
In prior art systems it is known to provide a tightening system which comprises a holder and a rod. The holder fits at its distal end onto an insert which in turn fits into cavities in the upper face of the interspinous implant. In this way the holder can be fitted to the implant. The holder has at its proximal end a handle with apertures through which the rod can pass. The rod has an aperture at its distal end. In use the holder is fitted to the implant and the rod is passed through the aperture in the holder and down to the level of the implant. The band is passed through the aperture in the rod and the rod is rotated relative to the holder to tighten the band. This represents an improvement over using manual tightening or non-specific tools but it still difficult to use. One reason for this is that it is difficult to ensure the rod is in the right place to contact the band and to pull it in a direction in which it can be tightened. Another reason is that the rotational force applied during tightening can easily be transferred to the whole implant rather than just the band, so can cause the implant to rotate in the implant body away from the position in which the surgeon installed it. In addition, as noted above, a removable fixing member can become detached from the implant body during tightening.
Accordingly, the present invention also aims to address some of the problems encountered in the tightening step of installing interspinous implants. In particular, the present invention aims to provide a tightening system that improves the ease of implant installation thus making it more reliable, reproducible and safer.